I might be imagining it but I swear bigger wheels roll faster.
My wife and I doing a smooth dirt road downhill, neither of us pedaling, I'm way faster than her on 29 vs 26 tires is the most obvious example.
I'd do whatever I could to preserve wheel diameter.
I rode the White Rim trail in Moab on 700x38, the tires were fine but I should have used lower gearing. I think you can survive your commute on 40's.